Principles For A Mars Transport System

The following text, by Stephen Ashworth FBIS, has been presented at the British Interplanetary Society’s “Ways to Mars” symposium, held on 19 November 2008 at the Society’s London headquarters. Its main points:

— Most of the mass needed for an Earth-Mars transport system consists of propellants and life support materials, and that is already in space, and already in orbits very close to the ones which we need;

— But this near-Earth asteroidal resource is completely invisible to the space agency paradigm of space exploration, because [the paradigm] excludes the construction of permanent human activity in space.

Transport for Areopolis Or: “Implications of the Choice of Economic Paradigm for Strategies of Manned Access to the Moon and Mars” by Stephen Ashworth

When considering human access to Mars, it seems to me that there are two key points which need to be taken into account, but which are often ignored. I shall offer you these two points very shortly.

Designs for manned missions to Mars typically involve assembling in low Earth orbit a spaceship weighing several hundred to over a thousand tonnes.

For example, each Troy spacecraft, which we shall be hearing more about this afternoon, weighs nearly 800 tonnes to carry 6 astronauts. The “space lego” nuclear powered Mars mission uses two ships of 240 tonnes each, thus a total of 480 tonnes in low Earth orbit. [I was wrong — this turns out to come to a total of 955 tonnes.] The “magic” Mars mission requires five Energiya launches, thus probably weighs 4 to 500 tonnes when ready to go.

Meanwhile the official European Space Agency design study for a Mars mission proposed a ship, again for 6 astronauts, which required 20 Energiya launches for every single Mars departure. These launches would build up a ship weighing 1357 tonnes at departure.

The Mars ship in low Earth orbit thus weighs between about 50 tonnes and about 200 tonnes per astronaut on board. Launching such large masses into orbit for the benefit of so few people is one reason why manned Mars exploration is hopelessly uneconomic.

At present-day cargo rates to low Earth orbit of $10 million per tonne, this is a billion dollars per astronaut, plus the cost of the Mars hardware itself. Even at spaceplane rates, which may fall to as low as $10 thousand per tonne, this is still a million dollars per astronaut, plus the cost of the hardware.

At these rates, there will not be many people going to Mars.

Let me show you an Earth-Mars transfer orbit.

Orbit of Earth, orbit of Mars, and an elliptical orbit which intersects both of them

Here is an orbit which reaches out from Earth to pass the orbit of Mars. It has about the same size and shape as the orbit of an Earth-Mars cycler, such as the ones being studied by Buzz Aldrin and his collaborators.

It might therefore be the orbit of a future manned Mars vehicle. But that’s not what I drew. What I’m showing you here is the orbit of minor planet 4660 Nereus.

The concept of an interplanetary cycler, which repeatedly encounters Earth and Mars, goes back to the early 1980s. Alan Friedlander and John Niehoff first proposed setting up long-lived space habitats which remain permanently in interplanetary space. These would periodically be used for transporting people between Earth and Mars. Relatively small ferry spacecraft would complete the transport chain between the cycler and a local parking orbit or planetary surface.

In 1985 Buzz Aldrin added the concept of a gravity assist at each planetary flyby. This technique allows a cycler to stay in phase with the relative motion of Earth and Mars. It enables it to offer passage between these planets once every 2.14 years, the Earth-Mars synodic period.

A great number of near-Earth asteroids, such as 4660 Nereus, resemble natural Earth-Mars cyclers. A proportion of them are believed to be carbonaceous chondrites, containing water and other volatiles. Water in space is of incalculable value as a feedstock for propellant manufacture, as a near ideal substance for radiation shielding, and for other life support functions.

I have checked the online listings of near-Earth asteroids published by the Minor Planet Center. Applying quite stringent orbital criteria, I found a total of 56 Amor and Apollo asteroids which behave like natural Earth-Mars cyclers. New ones are being discovered all the time — for example, of those 56, ten were only identified this year.

Now to my two key points.

Firstly: most of the mass needed for an Earth-Mars transport system consists of propellants and life support materials. That mass is already in space, and already in orbits very close to the ones which we will need to reach and return from Mars. It does not need to be launched from Earth. It can be mined in situ.

So why is hardly anybody getting excited about this? Why does it not form the basis of the Constellation programme, or of the recent ESA or Russian design studies, or even the magic, the trojan or the space lego Mars missions?

Because of my second point: the asteroidal resource is completely invisible to the space agency paradigm of space exploration. That mode of planning excludes the possibility of systematic use of natural in-space materials, and it excludes the construction of permanent infrastructure on Earth-Mars cycler orbits. It will not contemplate anything that suggests permanent human activity in space.

I think we can identify two broadly contrasting attitudes to transport infrastructure.

The heroic paradigm is only interested in special missions of heroic exploration. This is the space agency mode of thinking. Its prime goal is national presige, under a fig-leaf of science, spinoff and educational inspiration. Think of the Apollo programme. Further back in history, think of Zheng He’s epic voyage of exploration around 1421, from a China which was about to close in on itself.

In contrast with the heroic paradigm, we can identify the systemic paradigm of transport infrastructure. The prime goals here are permanence, growth, and economic profitability. Think of the Cunard and White Star steamers which connected Britain with the Americas and the Empire from the mid-nineteenth to the mid-twentieth century.

Now obviously, since there is currently nobody on the Moon or Mars, the next people to travel there will of necessity be heroic government explorers. But the question we need to address is this: will their transport system be designed for cancellation, like Apollo, or will it be designed for growth, like Cunard?

Secondly, it will not be content with a single route — say, between Kennedy spaceport on Earth and a single base at Utopia Planitia on Mars. It will rather seek to foster a network of different routes among a number of different transport nodes. Those nodes may include an increasing number of space hotels, factories and laboratories, and lunar and martian bases.

Note particularly that the use of transport nodes allows in-space refuelling. This capability was regarded by early spaceflight theorists such as Hermann Oberth and Guido von Pirquet as essential if lunar and martian flights were to become achievable using chemical fuels.

Thirdly, a systemic space transport system will diversity its sources of propellants and life support materials, exploiting the transport nodes for in-space refuelling.

Fourthly, it will not be content with a pillar architecture, but will develop a pyramidal one. In a pillar architecture, one unique space station is succeeded by one unique Moon base, and that in turn by one unique Mars base. In a pyramid architecture, by contrast, it is growth in the use of space stations that supports the first Moon base, and growth in the use of Moon bases that supports the first Mars base.

Thus in impressionistic figures, if there are ten people on Mars, then we should expect to see at the same time at least a hundred people on the Moon, and at least a thousand on board stations in Earth orbit at any one time.

So we can now design a Mars transport system along the following principles:

— The long-haul journey is accomplished on modular interplanetary cycler stations, which are upgrades of stations in regular use as Earth-Moon cyclers, which are themselves upgrades of stations in regular use in low Earth orbit as hotels, factories and so on;

— The transport chain between Earth and the interplanetary cyclers is closed by short-range ferries, which are upgrades of ferries in regular use to connect with the Earth-Moon cyclers, which are themselves upgrades of ferries in regular use between Earth’s surface and low Earth orbit;

— The bulk of the development work that goes into the first Mars mission is carried out by commercial companies in pursuit of profitable business in space tourism, manufacturing and energy;

— As a result of growth in traffic in the Earth-Moon system, an in-space refuelling system based on near-Earth asteroidal water will become economically viable, vastly decreasing launch costs from Earth.

There may still be a heroic attempt to get to Mars in isolation from the development of such a space economy. If we are lucky, it will be like Apollo, and will be cancelled after the first few landings. If we are unlucky, it will be like the X-33 or Hermès spaceplanes, or like the Soviet Moon-landing programme, and be cancelled before its first landing.

Either way, it will not produce much progress towards sustainable human access to Mars. That can only be achieved by a systemic transport system, not a heroic one.

To conclude, I would remind you of my two key points:

— Most of the mass needed for an Earth-Mars transport system consists of propellants and life support materials, and that is already in space, and already in orbits very close to the ones which we need;

— But this near-Earth asteroidal resource is completely invisible to the space agency paradigm of space exploration, because it excludes the construction of permanent human activity in space.

[…] presented at the British Interplanetary Society symposium, 19 November 2008. Also see Maurizio Morabito’s blog for a discussion. Thanks to Mark Hempsell for pointing out an error in my rubble thickness […]